Electrochemical Method And Sensor For The Detection Of Traces Of Explosives

ABSTRACT

A system for highly sensitive electrochemical detection of trace nitro-aromatic compounds in air, uses a carbon or carbon/gold working electrode with a surface that is modified to increase the electron transfer kinetics of nitro-aromatic compounds, Chemical modifiers of the working electrode surface include amino-aromatic compounds such as aniline and its derivatives The detection method involves dissolving trace nitro-aromatic compounds in an electrolyte including aprotonic solvents, or dipolar solvents, in the electrochemical cell including a working electrode, a reference electrode and an auxiliary electrode. Voltage is varied across the working electrode and the reference electrode, and an electrical current is measured between the working electrode aid the auxiliary electrode. The measured electrical peak current is a sensitive indication of the concentration of the trace compounds. This invention is appropriate for portable, field-testing of trace explosive compounds in air.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/715,489 filed Nov. 19, 2003 by the present inventor.

FIELD OF THE INVENTION

The present invention relates to the detection of trace amounts ofexplosive materials such as nitro-aromatic compounds in air, using anelectrochemical measurement technique, and specifically to improving thesensitivity of the measurement of trace explosive materials, anddecreasing measurement time More specifically, the present inventionrelates to a method of explosives detection of low cost that is suitablefor portable field-testing.

BACKGROUND OF THE INVENTION

As a consequence of recent efforts to thwart the recent upsurge ininternational terrorism, there is an increased interest in the detectionof explosive materials. These materials include nitro-aromatic compoundsincluding 2,4,6-trinitrotoluene (TNT), dinitrotoluene (DNT) and similarderivatives.

Many detection methods have been used to detect explosive materials.These methods include gas and HPLC chromatography, x-ray scattering,neutron analysis, nuclear quadrupole resonance, and mass spectrometry(U.S. Pat. No. 6,571,649). These methods generally require expensive andsophisticated equipment, (e g. high vacuum), equipment that is notportable (e.g. cylinders of compressed gases), and/or have a complicatedsample preparation These techniques, are therefore, not appropriate forlow cost portable field-testing for trace explosive materials. A recentreview of some of these methods for explosives detection is “Explosivesdetection systems (EDS) for aviation security” (Singh, S., SignalProcessing vol. 83, 2003, p. 31-55).

Another known method for the detection of trace amounts of explosivematerials utilizes immunochemical sensors. For example, U.S. Pat. No.6,573,107 is directed towards the immunochemical detection of explosivesubstances in the gas phase using surface plasmon resonancespectroscopy. Immunochemical detection methods potentially offer highselectivity and high sensitivity.

Electrochemical detection refers to the use of electrodes, immersed inan electrolyte, and connected to an instrument that varies the voltageapplied to the electrodes. The instrument measures the current flowbetween the electrodes Typically, the electrode potential is varied; andan electric current flows between the electrodes that is characteristicof the presence of electrochemical active substances in the electrolyte.The magnitude of the current is proportional to the concentration of theelectrochemical active substances. It is well known that TNT and othernitro-aromatic compounds are reduced electrochemically at the cathodeand may be detected by electrochemical detection. Wang et al. (AnalyticaChimica Acta, vol. 485 (2003) p. 139-144) reported the monitoring of TNTin natural waters using an electrochemical technique. They reported ameasurement sensitivity of 0.003 μA/ppb of TNT in natural seawater. Thissensitivity level was achieved by Wang et al. by subtracting thebackground signal, in natural seawater not contaminated by TNT, causedby the reduction of dissolved oxygen. The applicant reported (ReviewsAnalytical Chemistry vol. 18 no. 5, 1999, p. 293) the use of carbon/Hgfilm electrode materials in an aqueous solvent. This electrode materialwas successful to minimize the background by separating the atmosphericO₂ background current from the TNT current, however the sensitivityreported was only ˜0.7 μA/μM (˜0.003 μA/ppb) and was comparable to thesensitivity reported by Wang. Despite these positive developments in theprior art, the sensitivity is still insufficient, and the kinetics ofthe TNT reduction reaction are too slow to achieve a practical portablefield test for trace explosive materials. A practical electrochemicalsensor for trace explosive materials should have high sensitivity, ashort measurement time and in addition a way of cleaning the electrodesrapidly to perform further testing,

There is thus a widely recognized need for an electrochemical method andsensor for the detection of traces of explosives, and it would be highlyadvantageous to have an electrochemical method and sensor for thedetection of traces of explosives, with high sensitivity, and fastreaction kinetics.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a system forelectrochemical assay of nitro-aromatic compounds, including: (a) aworking electrode having a surface modified with a chemical thatincreases electron transfer kinetics of the nitro-aromatic compounds.

Preferably, the chemical that increases the electron transfer kineticsis an aromatic compound, for example al amino-aromatic compound, analkyl-aniline compound, a halide derivative of an alkyl aniline compoundand/or an hydroxyl-aniline compound. Most preferably, the chemicalmodifier is phenylene-diamine, diphenylene-diamine,diphenylene-triamine, or aniline.

Preferably, the working electrode contains elemental carbon or gold; theworking electrode includes submicron particles and the elemental gold isa coating on the electrode surface. Preferably, the working electrodeincludes carbon paper.

Preferably, the system includes in addition, (b) an electrolyte fordissolving the nitro-aromatic compounds, and the electrolyte is chosento minimize background current resulting from oxygen reduction.

Preferably, the electrolyte includes an aprotonic solvents, and/ordipolar solvents; such as dimethylformamide, acetonitrile, propylenecarbonate and optionally also a solvent such as ethanol, propanol,ethylene-glycol, and/or propylene-glycol.

Preferably, the system further includes (c) a mechanism for inputtingair suspected to include the nitro-aromatic compounds, into theelectrolyte in order to dissolve the nitro-aromatic compounds in theelectrolyte.

According to the present invention, there is provided an electrochemicalmethod of assaying trace compounds in air, including the steps of (a)dissolving the trace compounds in an electrolyte that includes aprotonicsolvents, and/or dipolar solvents; (b) immersing a working electrode inthe electrolyte; (c) applying a varying potential to the workingelectrode; and (d) measuring an electrical current consequent to thevarying potential, thereby providing measurement results indicative of aconcentration of the trace compounds.

Preferably, after measurement, the electrochemical method includes (e)regenerating the working electrode by applying a negative potential tothe working electrode.

Preferably, the dissolving of trace compounds is performed by bubblingair containing the trace compounds through the electrolyte.

Preferably, the electrochemical method includes, prior to dissolving thetrace compounds in the electrolyte, the steps of: (f) measuring abackground electrical current, while applying a varying potential,thereby obtaining background current results; and (g) subtracting thebackground current results from the measurement results, therebyobtaining calibrated measurement results.

Preferably, the electrolyte used in the electrochemical method includesdimethylformamide, acetonitrile, and/or propylene carbonate; optionallyalso ethanol, propanol, ethylene-glycol, and/or propylene-glycol andpreferably the electrolyte has pH greater than 7. Preferably, theelectrochemical method includes preconditioning the working electrodethereby increasing electron transfer kinetics of the trace compounds.The preconditioning modifies the working electrode with a chemical suchas amino-aromatic compounds, alkyl-aniline compounds, halide derivativesof alkyl aniline compounds and/or hydroxyl-aniline compounds. Accordingto the present invention, there is provided an electrochemical method ofassaying nitro-aromatic compounds in air, including the steps of: (a)dissolving the nitro-aromatic compounds in an electrolyte that includesan aprotonic solvent, and/or a dipolar solvent; (b) immersing a workingelectrode in the electrolyte; (c) applying a varying potential to theworking electrode; (d) measuring an electrical current consequent to thevarying potential, thereby providing measurement results, indicative ofa concentration of the nitro-aromatic compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein;

FIG. 1 is a drawing of an electrochemical cell used to sense traceexplosive materials, according to the present invention;

FIG. 2 is a graph of background current measured in an electrochemicalcell using both a conventional aqueous solvent and an electrolyte of thepresent invention;

FIG. 3 is a graph showing the sensitivity of the TNT detection, using anelectrolyte of the present invention, and a conventional workingelectrode.

FIG. 4 is a graph showing the measurement sensitivity at pH=4 with amodified electrode, according to the present invention;

FIG. 5 is a graph showing the measurement sensitivity with a modifiedelectrode and with electrolyte of pH=9, according to the presentinvention;

FIG. 6 is a graph showing the measurement sensitivity with an electrodesurface further modified according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is an electrochemical method and sensor for thedetection of traces of explosives. Specifically, the present inventioncan be used for the detection of trace amounts in air of nitro-aromaticcompounds including 2,4,6-trinitrotoluene (TNT), dinitrotoluene (DNT)and similar derivatives.

The principles and operation of an electrochemical method and sensor forthe detection of traces of explosives, according to the presentinvention, may be better understood with reference to the drawings andthe accompanying description.

Referring now to the drawings, FIG. 1 illustrates an electrochemicalcell 101 including a working electrode 103, a reference electrode 105,an auxiliary electrode 106, an electrolyte 109, an air inlet 107 thatpasses air into the cell through a perforated tube 108 allowing air tobubble through electrolyte 109, and an air outlet 115 that lets air outof the cell. Reference electrode 105 is an Hg/HgCl electrode thatincludes an element 111 to protect reference electrode 105 from airbubbles. Electrolyte 109 is a solvent or a mixture of solvents includingtrace materials dissolved in the solvent(s). These trace materials,including nitro-aromatic compounds dissolved in the solvent(s), areadmitted to the solvent mixture from the air using air inlet 107 anddissolved in electrolyte 109 by bubbling the air through electrolyte109. Air is output through air outlet 115. A metal screen 113 is used toprevent electrolyte 109 from escaping through air outlet 115. Voltage isapplied between working electrode 103 and reference electrode 105. Acurrent is measured which flows between reference electrode 103 andauxiliary electrode 106, as a result of oxidation-reduction reactions onthe electrode surfaces in electrolyte 109. Working electrode 103 isprepared by a technique of galvanic Au plating of carbon on an ordinarycarbon paper surface in an aqueous solution of HAuCl, K₄Fe(CN)₆, andNa₂CO₃ at current density 1ma/cm². The carbon particles are of typicaldimension 0.1-1 μm. The carbon paper with density 0.4-0.8 g/cc, partnumber P2 or P3 was supplied by E-TEK Inc. (Somerset, N.J., USA). Thegold layer deposited on the carbon particles is of approximatethickness, 0.30-0.60 nm. All reagents used were obtained fromSigma-Aldrich (USA).

Voltammetric measurements were performed using a CV-50W potentiostat ofBioanalytical Systems Inc. (West Lafayette, Ind., USA). Measurementswere performed in the differential pulse (DIP) mode. A backgroundcurrent was measured using background electrolyte 109 before traceelements are introduced into electrolyte 109. Trace elements are thenintroduced into electrolyte 109 through air inlet 107 using a standardair pump (not shown in FIG. 1) with throughput 1500 ml/min for 20 sec.The current measurement is then performed and the background current issubtracted from the measured current to yield the measured results. Thepotential range is from −150 mV to −500 mV; the scan rate is from 30 to50 mV/sec. Electrochemical active substances in the air includingnitro-aromatic compounds (e.g. TNT) are dissolved in electrolyte 109 andare detected electrochemically.

FIG. 2 is a graph of DIP voltammetric data measured with C/Au workingelectrode 103. For data trace 201, electrolyte 109 is 0.1M KCLO₄ inwater with pH 9.0, For data trace 203, electrolyte 109 is 0.1M KCLO₄ ina mixture of water, ethanol, and acetonitrile (1:1:1 v/v), with pH 9.0.FIG. 2 shows a window of about 250 mV, between potentials −0.5 and 0.6Vwhere trace 302 has a lower background current. The higher backgroundcurrent of trace 201 is attributed to dissolved gaseous oxygen thatinterferes with the measurement. Consequently, the mixed electrolyte oftrace 203 is preferred.

FIG. 3 is a graph of DIP voltammetric data measured using anelectrochemical cell shown in FIG. 1, according to the presentinvention. However, the data of FIG. 3 were measured with a workingelectrode C/Au 103 that was immersed in pure dimethylsulfoxide (DMSO)for 5 minutes at room temperature. Trace 301 is the backgroundvoltammetric data with electrolyte 109 of pH 4 consisting ofwater/ethanol/acetonitrile 1:1:1 (v/v). Trace 302 shows the voltammetricdata with 200 ug/l(200 ppb) TNT dissolved in electrolyte 109. Eachsubsequent trace 302 to 304 shows voltammetric data each with anadditional 200 ug/l (200 ppb) TNT dissolved in electrolyte 109. The TNTmeasurement sensitivity is 0.003 μA/ppb.

FIG. 4 is a graph of DIP voltammetric data measured in accordance withthe present invention. Working electrode 103 is preconditioned byimmersing working electrode 103 in a 2% solution of aniline in DMSO for5 minutes at room temperature. Electrolyte 109 is the same as that ofFIG. 3, of pH 4, consisting of water/ethanol/acetonitrile 1:1:1 (v/v).Trace 401 shows the background voltammetric data. Each subsequent trace402 to 407 shows voltammetric data each with an additional 60 μg/l (60ppb) TNT dissolved in electrolyte 109. The sensitivity of themeasurement is shown in the graph of inset 410, in which the abscissa isthe TNT concentration (ppb) and the ordinate is the measured peakcurrent in μA. The measured sensitivity is 0.11 μA/ppb. These datacompared with the data of FIG. 3 show that the surface modification ofworking electrode 103 with aniline significantly improves themeasurement sensitivity of TNT.

FIG. 5 is a graph of DIP voltammetric data measured in accordance withthe present invention. The surface of working electrode 103 is immersedas in the measurement of FIG. 4, in a 2% solution of aniline in DMSO.Electrolyte 109 is similar to that used in the measurement of FIG. 4,consisting of water/ethanol/acetonitrile 1:1:1 (v/v), but with pH 9. ThepH was achieved using Merck buffer capsules, no. CPM90L4 pH=9. Traces501 to 505 show the calibrated data after subtracting the backgrounddata. Traces 501 to 505 show the calibrated data each with an additional10 μg/l(10 ppb) dissolved in electrolyte 109, starting with 10 μg/l intrace 501. Graph inset 510 shows a sensitivity of 0.50 μA/ppb. Thesedata compared with the measurement of FIG. 4 show that the high pHincreases the measurement sensitivity.

FIG. 6 is a graph of DIP voltammetric data measured in accordance withthe present invention. The surface of working electrode 103 ispreconditioned by immersing working electrode 103 in a 3% solution ofaniline in DMSO for 5 minutes at room temperature. Electrolyte 109 isthe same as that of the measurement of FIG. 5, consisting ofwater/ethanol/acetonitrile 1:1:1 (v/v), with pH 9. Trace 601 shows thebackground data. Traces 602 to 606 show the measured data withoutsubtracting the background data. Traces 602 to 606 show the measureddata, each when an additional 15 ppb of TNT is dissolved in electrolyte109, starting with 15 ppb of TNT in trace 602. The measured sensitivityis shown in graph inset 610. The measured sensitivity is 0.66 μA/ppb,showing that the stronger aniline treatment significantly improves thesensitivity.

The measurement of nitro-aromatic compounds, according to the presentinvention requires about 30-40 sec. After the detection ofnitro-aromatic compounds, working electrode 103 is regenerated, ifnecessary, using a high negative potential about −1000 to −1300 mV.Regeneration time is about 5 seconds. Therefore, the electrochemicaltechnique, according to the present invention, is suitable for rapid andportable field-testing of nitro-aromatic compounds. The experimentalresults show that the use of solvents which are aprotonic, organicdipolar, or have a high dielectric constant, as part of electrolyte 109,are expected to improve the detection sensitivity, according to thepresent invention. These solvents include acetonitrile,dimethyl-formamide and propylene-carbonate and mixtures thereof. Otherpolar solvents such as ethanol, propanol, ethylene-glycol, andpropylene-glycol may be added as diluents. A preferred diluent has ahigh boiling point and is stable against evaporation. The electrodeused, according to the present invention, is manufactured from ordinarycarbon paper. Carbon paper is readily available, of low cost andtherefore suitable for a disposable electrode. An electrode, including acarbon particle layer on other substrates including cloth or glass, willfunction in a similar way.

The experimental results furthermore show that the chemical modificationof a, carbon or carbon/gold electrode, with compounds similar toaniline, preconditions the electrode surface to increase the sensitivityof the measurement, according to the present invention. These compoundsinclude aromatic compounds containing amino groups, includingderivatives that are mono-alkyl (e.g. methyl, ethyl, propyl, . . . ),di-alkyl or tri-alkyl. The chemical modifiers, according to the presentinvention include aromatic-aniline compounds such as, phenylene-diamine,diphenylene-diamine, diphenylene-triamine and similar compounds andderivatives. This chemical modification is necessary because thereduction of nitro-aromatic compounds has slow kinetics. Themodification successfully increases the electron transfer rate from thesolution to electrode 103 and therefore increases the sensitivity of themeasurement and lowers the detection limit. Other modifiers thatincrease electron transfer, according to the present invention, includecompounds such as J. Meisenheimers complexes, known to mediate electrontransfer, and other nitro-amine complexes; and alkyl-aniline and itsderivatives; and halide derivatives of alkyl aniline compounds as wellas hydroxyl-aniline compounds.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

1. An electrochemical method for assaying trace nitro-aromaticcompounds, the method comprising the steps of: (a) providing anelectrolyte including a mixed solvent of water and an organic solventselected from the group consisting of aprotonic solvents and organicdipolar solvents; (b) immersing a working electrode in said electrolyte,said working electrode having a surface including at least one elementselected from the group of carbon and gold; (c) applying a varyingpotential to said working electrode; and (d) measuring an electricalcurrent consequent to said varying potential, thereby providingmeasurement results indicative of a concentration of the nitro-aromaticcompound.
 2. The electrochemical method, according to claim 1, furthercomprising the step of: (e) regenerating said working electrode byapplying a negative potential to said working electrode.
 3. Theelectrochemical method, according to claim 1, further comprising thestep of: (e) dissolving the nitro-aromatic compounds in said electrolyteby bubbling air containing the nitro-aromatic compounds through saidelectrolyte.
 4. The electrochemical method, according to claim 1,further comprising the step of: (e) measuring a background electricalcurrent, while said applying a varying potential, thereby obtainingbackground current results.
 5. The electrochemical method, accord toclaim 4, further comprising the step of: (f) subtracting said backgroundcurrent results from said measurement results, thereby obtainingcalibrated measurement results.
 6. The electrochemical method, accordingto claim 1, wherein organic solvent is selected from the groupconsisting of dimethylformamide, acetonitrile, propylene carbonate. 7.The electrochemical method, according to claim 6, wherein said organicsolvent is selected from the group consisting of ethanol, propanol,ethylene-glycol, and propylene-glycol.
 8. The electrochemical method,according to claim 1, wherein said electrolyte has a pH greater than 7.9. The electrochemical method, according to claim 1, further comprisingthe step of, prior to said immersing: (e) preconditioning said workingelectrode by treatment thereof with a monomeric amino-aromatic compounddissolved in an organic polar solvent.
 10. The electrochemical method,according to claim 9, wherein said preconditioning includes modifying asurface of said working electrode with a chemical selected from thegroup consisting of amino-aromatic compounds, alkyl-aniline compounds,halide derivatives of alkyl aniline compounds and hydroxyl-anilinecompounds.
 11. An electrochemical cell comprising the electrolyte andthe working electrode for performing the method steps of claim
 1. 12. Anelectrochemical method of assaying nitro-aromatic compounds in air,comprising the steps of: (a) providing an electrolyte including a mixedsolvent of water and an organic solvent selected from the groupconsisting of aprotonic solvents and organic dipolar solvents; (b)immersing a working electrode in said electrolyte, said workingelectrode having a surface pre-treated thereof with a monomericamino-aromatic compound dissolved in an organic polar solvent; (c)applying a varying potential to said working electrode; (d) measuring anelectrical current consequent to said varying potential, therebyproviding measurement results indicative of a concentration of thenitro-aromatic compound.
 13. An electrochemical cell comprising theelectrolyte and the working electrode for performing the method steps ofclaim 12.